Bias Network

ABSTRACT

An HF-line includes parallel branches with bias terminals, to which a direct voltage source is respectively connected via a first inductor and a second inductor. The inductors are inductively coupled to one another.

This application claims priority to German Patent Application 10 2008 044 845.1, which was filed Aug. 28, 2008 and is incorporated herein by reference.

TECHNICAL FIELD

The present invention pertains to a circuit for feeding a direct voltage into an HF-line.

BACKGROUND

In modern transceiver circuits for use in mobile telephones, particularly in the bands for GSM and UMTS, it is frequently required to supply the output mixer stage of the HF-line of the transceiver (“open collector” or “open drain”) with a direct voltage externally via the HF-output pin. A so-called bias network provided for this purpose feeds the open-collector or open-drain output stages of the transceiver. This bias network should be integrated into a TX filter module. In order to maintain the size of the module as small as possible and thusly remain competitive, the normally used circuit technology cannot be employed in this case.

In order to feed a direct voltage into an HF-line, it is common practice to utilize a Bias-T connector element as described, for example, by James R. Andrews, entitled “Broadband Coaxial Bias Tees” (Picosecond Pulse Labs, Application Note AN-1e, Revision 1, Copyright November, 2000, http://picosecond.com/objects/AN-01e.pdf). The direct voltage is fed into the central conductor of a coaxial cable via a resistor or an inductor. In this case, the resistance should be significantly higher than the impedance of the HF-line in order to prevent a load on the HF-line. At a higher power demand, the direct voltage is coupled into the HF-line via an inductor in order to prevent a voltage drop from occurring at a resistor.

If the bias network is used in a TX (transmission) filter module, the module is preferably arranged on an LTCC substrate (low-temperature cofired ceramics), and the circuit of the bias network should be integrated into the LTCC substrate. When using a coil as an inductor for connecting the direct voltage, relatively high inductance values are required for achieving a sufficient decoupling of the direct voltage supply line from the HF-line. This complicates the integration of the inductor into the LTCC substrate if predetermined maximum dimensions of the TX filter module need to be observed.

SUMMARY

The present invention discloses a bias network for use in a TX module and that can be integrated into an LTCC substrate.

In the bias network, the HF-line features two parallel branches that are respectively provided with a bias terminal. The direct voltage is respectively fed into the corresponding branch via an inductor, wherein the two inductors are inductively coupled to one another. The total inductance required is significantly reduced due to the inductive coupling of the inductors. The thusly achieved reduction of the space requirement for these components makes it possible to integrate the inductors into the LTCC substrate in the form of coils without exceeding the predetermined dimensions of the substrate. If a coupling of 100% is achieved, the required inductance is reduced by 50% in comparison with an embodiment in which the inductors are not coupled. The outputs of the two branches of the HF-line can be combined into a single output (single-ended output) by means of a filter, particularly a surface wave filter. The inputs of the two branches of the HF-line can be connected to a mixer output of a transceiver IC as a symmetric input. Several of these bias networks can be used for different frequency bands of the same TX module.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the bias network are described in greater detail below with reference to the enclosed figures.

FIG. 1 shows a circuit diagram of one exemplary embodiment of the bias network;

FIG. 2 shows a circuit diagram of another exemplary embodiment of the bias network; and

FIG. 3 shows a schematic circuit diagram of a TX module comprising bias networks.

The following list of reference symbols may be used in conjunction with the drawings:

-   -   11 First DC supply circuit     -   12 Second DC supply circuit     -   13 Third DC supply circuit     -   21 First filter     -   22 Second filter     -   23 Third filter     -   31 First adapter     -   32 Second adapter     -   33 Third adapter     -   41 First input pair     -   42 Second input pair     -   43 Third input pair     -   51 First output     -   52 Second output     -   53 Third output     -   61 First direct voltage source     -   62 Second direct voltage source     -   63 Third direct voltage source     -   A Input of first branch     -   A′ Output of first branch     -   A1 Bias terminal of first branch     -   A2 Bias terminal of second branch     -   B Input of second branch     -   B′ Output of second branch     -   C1 First capacitor     -   C2 Second capacitor     -   C3 Third capacitor     -   F Filter     -   HF1 First branch of HF-line     -   HF2 Second branch of HF-line     -   L1 First inductor     -   L2 Second inductor     -   OUT Output     -   S LTCC substrate     -   TX TX module     -   V_(BIAS) Direct voltage source

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows a circuit diagram of one exemplary embodiment of the bias network with coupled inductors. The HF-line comprises a first branch HF1 with an input A, an output A′ and a bias terminal A1 and a second branch HF2 that lies parallel to the first branch and has an input B, an output B′ and a bias terminal A2. The direct voltage is fed from a direct voltage source V_(BIAS) into the branches of the HF-line via the inductors L1, L2. The direct voltage can be fed, in particular, to a transceiver connected to the input side in this fashion. In this embodiment, capacitors C1, C3 are provided for separating the direct voltage at the outputs A′, B′. The capacitor C2 is preferably provided for short-circuiting the high frequency referred to the ground and thusly separating the direct voltage and the alternating voltage in the circuit. The inductors L1, L2 are inductively coupled to one another such that the inductances only need to have low values in comparison with conventional circuits used for feeding a direct voltage into an HF-line. This makes it possible to use circuit components with relatively small dimensions.

FIG. 2 shows a circuit diagram of another exemplary embodiment of the bias network, in which the capacitors C1, C3 in the branches of the HF-line are replaced with a filter F. The filter F may comprise a surface wave filter. The inductors L3 and L4 respectively serve for adapting the impedances between a component connected to the inputs such as, for example, a transceiver and the filter F or between the filter F and a component connected to the output OUT such as, for example, a power amplifier. In this exemplary embodiment, the outputs A′, B′ of the branches HF1, HF2 of the HF-line are combined into a single output OUT (single-ended output) by means of the filter F. Both described exemplary embodiments make it possible to supply the mixer end stage of a transceiver IC connected to the input side with an external direct voltage (V_(BIAS)).

A TX module realized with the bias network may be designed for more than one frequency band. Such an example is illustrated in the circuit diagram according to FIG. 3. The TX module TX may be integrated, for example, on an LTCC substrate S as indicated by the frame illustrated in the circuit diagram according to FIG. 3. The exemplary TX module is designed for three different frequency bands. One bias network is provided for each band. The bias networks respectively comprise a pair of symmetric inputs 41, 42, 43, a DC supply circuit 11, 12, 13, a filter 21, 22, 23, an adapter 31, 32, 33, an output 51, 52, 53 and a direct voltage source 61, 62, 63. The DC supply circuit 11, 12, 13 respectively comprises, for example, a partial circuit of the circuit illustrated in FIG. 2 and comprises the components L1, L2 and C2. Each of the direct voltage sources 61, 62, 63 corresponds to the direct voltage source V_(BIAS) of the circuit according to FIG. 2, wherein several direct voltage sources may be provided for different values of the direct voltage. The input pairs 41, 42, 43 respectively correspond to the pair of inputs A, B in the exemplary embodiment according to FIG. 2. The filters 21, 22, 23 respectively comprise, for example, the inductor L3 and the filter F of the circuit according to FIG. 2. The separation of the direct voltage from the outputs 51, 52, 53 can be realized by using filters 21, 22, 23 in the form of surface wave filters. The adapter 31, 32, 33 may be respectively realized in the form of an inductor L4 according to the circuit shown in FIG. 2. The number of bias networks in the TX module is basically arbitrary. Consequently, several transceivers can be connected and respectively supplied with a direct voltage that is adapted to the corresponding frequency band. 

1. A bias network comprising: a direct voltage source node; an HF-line that features a first branch with a first bias terminal and a second branch with a second bias terminal, the second branch lying in parallel to the first branch; a first inductor; a second inductor that is inductively coupled to the first inductor; a first connection between the direct voltage source node and the first bias terminal of the first branch produced via the first inductor; and a second connection between the direct voltage source node and the second bias terminal of the second branch produced via the second inductor.
 2. The bias network according to claim 1, wherein the bias network is part of a TX module.
 3. The bias network according to claim 2, wherein the TX module comprises an LTCC (low temperature co-fired ceramic) substrate and wherein the bias network is integrated into the LTCC substrate.
 4. The bias network according to claim 2, wherein the TX module comprises HF-lines for different frequency bands and each HF-line has two branches that are connected to a direct voltage source via inductors that are inductively coupled to one another.
 5. The bias network according to claim 1, wherein the first and second branches of the HF-line are combined on an output side by means of a surface wave filter.
 6. A bias network comprising: a first branch of an HF-line, the first branch having a first input, a first output and a first bias terminal between the first input and the first output; a second branch of the HF-line, the second branch having a second input, a second output and a second bias terminal between the second input and the second output; a direct voltage source node; a first inductor coupled between the direct voltage source node and the first bias terminal of the first branch of the HF-line; and a second inductor coupled between the direct voltage source node and the second bias terminal of the second branch of the HF-line.
 7. The bias network of claim 6, further comprising a first capacitor coupled between the direct voltage source node and a ground node.
 8. The bias network of claim 7, further comprising: a second capacitor coupled between the first bias terminal and the first output of the first branch of the HF-line; and a third capacitor coupled between the second bias terminal and the second output of the second branch of the HF-line.
 9. The bias network of claim 7, further comprising a filter coupled between the first output and the second output, the filter coupled to a single-ended output.
 10. The bias network of claim 9, further comprising: a third inductor coupled between the first output and the second output; and a fourth inductor coupled between the filter and the single-ended output.
 11. The bias network of claim 9, wherein the filter comprises a surface acoustic wave filter.
 12. The bias network of claim 6, further comprising a surface acoustic wave filter coupled to the first and second outputs.
 13. A TX module comprising: a substrate; a first direct voltage source node at the substrate; a second direct voltage source node at the substrate; a first bias network disposed on the substrate, the first bias network comprising: first and second inputs; a first DC supply circuit comprising a first inductor coupled between the first direct voltage source node and the first input, a second inductor coupled between the first direct voltage source node and the second input, and a capacitor coupled between the first direct voltage source node and a ground node; a first filter coupled to the first DC supply circuit; a first output coupled to the first filter, the first output comprising a first output of the TX module; and a second bias network disposed on the substrate, the second bias network comprising: third and fourth inputs; a second DC supply circuit comprising a third inductor coupled between the second direct voltage source node and the third input, a fourth inductor coupled between the second direct voltage source node and the fourth input, and a second capacitor coupled between the second direct voltage source node and the ground node; a second filter coupled to the second DC supply circuit; a second output coupled to the second filter, the second output comprising a second output of the TX module.
 14. The TX module of claim 13, wherein the substrate comprises an LTCC (low temperature co-fired ceramic) substrate.
 15. The TX module of claim 13, wherein the first filter comprises a surface acoustic wave filter and wherein the second filter comprises a surface acoustic wave filter.
 16. The TX module of claim 15, wherein the first filter further comprises a fifth inductor coupled in parallel with the first filter and a sixth inductor coupled in parallel with the second filter.
 17. The TX module of claim 13, wherein the first bias network further comprises a first adapter coupled between the first filter and the first output and wherein the second bias network further comprises a second adapter coupled between the second filter and the second output.
 18. The TX module of claim 17 wherein the first adapter comprises an inductor and wherein the second adapter comprises an inductor.
 19. The TX module of claim 13, wherein the first bias network is part of a first HF-line that operates at a first frequency and the second bias network is part of a second HF-line that operates at a second frequency.
 20. The TX module of claim 13, further comprising a third bias network disposed on the substrate, the third bias network comprising: fifth and sixth inputs; a third DC supply circuit comprising a fifth inductor coupled between a third direct voltage source node and the fifth input, a sixth inductor coupled between the third direct voltage source node and the sixth input, and a third capacitor coupled between the third direct voltage source node and the ground node; a third filter coupled to the third DC supply circuit; a third output coupled to the third filter, the third output comprising a third output of the TX module. 